Beginner DENEB assistance

Hi all I am new to Deneb and am trying to create a heat map. The number of columns and rows are dependent on slicer selection.

I am trying to make the column and height a fixed size. Atm they dynamical change size. This is what I have tried so far. 

Any help or pointers in the right direction would be greatly appreciated. 

{
  "data": {
    "name": "dataset"
  },
  "encoding": {
    "y": {
      "field": "Course Name",
      "type": "ordinal",
      "axis": {
        "labelAngle": 0
      }
    },
    "x": {
      "field": "Staff Name",
      "type": "ordinal",
      "axis": {
        "orient": "top",
        "labelAngle": -45
      }
    }
  },
  "layer": [
    {
      "mark": "rect",
        "width": 500,
  "height": 500,
      "encoding": {
        "color": {
          "condition": [
            {"test": "datum.Combined === 1", "value": "green"},
            {"test": "datum.Combined === 2", "value": "orange"},
            {"test": "datum.Combined === 3", "value": "red"},
            {"test": "datum.Combined === 4", "value": "white"}
          ]
        }
      }
    }
  ],
  "config": {
    "axis": {"grid": true, "tickBand": "extent", "gridWidth": 2.5}
  
  }
}